A pressing force sensor of interest to the present invention is described, for example, in JP 3855410 B1 (Patent Document 1). Patent Document 1 describes a strain gauge with a resistor formed by printing resistive paste such as carbon paste on an insulating substrate, for detecting a mechanical strain exerted on the resistor as a change in resistance value.
In the strain gauge described in Patent Document 1, as the insulating substrate, one having relatively high rigidity such as a phosphor-bronze plate with an insulated surface is used, and a unimorph structure in which the resistor is formed on the insulating substrate having the relatively high rigidity is adopted. The resistor is subjected to stress either in a compressive direction or in a tensile direction.
However, the strain gauge described in Patent Document 1 has problems as below.
Due to the unimorph structure, most of a pressing force is expended for deformation energy of the insulating substrate. Further, since the resistor is subjected to both compressive and tensile stresses, symmetrical changes in resistance are required with respect to stresses in both the directions.
Patent Document 1: JP 3855410 B1